1. Field of the Invention
The present invention relates to a novel curing agent for epoxy resins. More specifically, the present invention relates to a curing agent for epoxy resins, wherein the curing agent is prepared by reacting 2,5- and 2,6-bis(aminomethyl)-bicyclo[2.2.1]heptanes(norbornanediamine; hereinafter abbreviated as NBDA) with an epoxy resin.
2. Description of the Related Art
Aliphatic polyamines have widely been used as a curing agent for epoxy resins because of their capability of curing epoxy resins rapidly even at a room temperature, as well as their ease in handling due to being in a liquid state in most cases. However, aliphatic polyamines themselves generally have an unpleasant odor and cause skin irritation, and a cured product formed as a result of reaction between an aliphatic polyamine with an epoxy resin absorbs carbon dioxide in the air to cause the phenomenon of whitening or so-called amine blushing.
In addition, compared with the aliphatic polyamines, alicyclic polyamines are known to have such advantages as low vapor pressure and weak skin irritation. This, however, does not thoroughly compensate for the above-mentioned drawbacks of the aliphatic polyamines. For example, NBDA, one of the allcyclic polyamines, is disclosed in German Patent No. 3,202,491. According to this patent, unmodified NBDA rapidly cures bisphenol diglycidyl ether epoxy resins at a low temperature (50.degree. C. or lower) and the resultant cured products exhibit good acid resistance. However, when used as a curing agent without modification, NBDA has nearly the same drawbacks as those of aliphatic polyamines and cannot be used an a practical curing agent. While having low basicity and weak skin irritation, aromatic polyamines are low in a curing rate and are difficult in handling due to being in solid state in many cases.
In this respect, modified amine curing agents are widely used, where in general, an epoxy resin is made addition to an aliphatic polyamine. However, the conventional modified amine curing agents for epoxy resins have drawbacks such as the resultant cured products have insufficient water resistance and, in many cases, surface tackiness. Also investigated are curing agents which are prepared by adding epoxy resins to aromatic polyamines; however, they still have several drawbacks such as a low curing rate at a low temperature, low stability due to crystallization, yellowing at the surface of the resultant cured products, and low flexibility of the cured products.
Japanese Patent Application Laid-open Nos. SHO54-4992 and HEI 4-80228 disclose a modified monoepoxy compound as a curing agent for epoxy resins, wherein the modified monoepoxy compound is prepared by reacting NBDA with a monoepoxy compound such as ethylene oxide, propylene oxide, butyl glycidyl ether and the like.
In the former disclosure, the resultant modified monoepoxy compound is evaluated to have an effect of lengthening the pot life, i.e. to reduce the curing rate at a room temperature, and the epoxy resin films heat-cured by using the compound exhibit good impact resistance, flexibility, chemical resistance, etc. However, the modified monoepoxy compound provides a low curing rate at a room temperature, and the resultant cured products have insufficient properties as to gloss, water resistance, chemical resistance, etc. The reason for such drawbacks would be that, through the reaction of NBDA with a monoepoxy resin, most of the primary amino groups in NBDA change into the secondary ones to reduce the reactivity of the resultant modified product with epoxy resins and the crosslinking density of the cured products.
In the latter disclosure, the resultant modified monoepoxy compound is evaluated to have low viscosity and the epoxy resin films cured at a room temperature by using the compound exhibit excellent adhesion and water resistance. However, the modified monoepoxy compound provides a low curing rate at room temperature, and the resultant cured films have insufficient properties such as chemical resistance, etc.